For starters, I'd never use it for consumption - ever.
But nothing degrades chemicals like bleach, sunlight, and time. I think with enough work you may be able to get it to the point you could make a grey-water tank out of it. Question is - what does a new tank cost and is it worth the effort?
I have worked for a lawn care company right after I got back from the dessert... I can tell you now that with the chemicals we used, you could rinse that thing out with bleach over 1000 times and it will still get you sick.. the chemicals made to kill the weeds and or to use as fertilizer is strong enough to hold a presence even after rinsing and years of sitting... The end result is you should never ever use it for consumption.